—Control in Authority.
The Border is a boundary separating Area X from the rest of the world. It is invisible to the naked eye, although it features an entrance that can be seen when looking at it frontally. Upon the slightest contact with the Border, objects and living beings disappear completely, as demonstrated in an experiment conducted by the Southern Reach before the First expedition, in which hundreds of white rabbits were rounded up near the Border, and herded through it.
Some Southern Reach scientists (mainly Mike Cheney) theorize that the Border and Area X are separate entities, and that the Border is either trying to contain Area X and prevent it from spreading or protecting it from outside interference, supported by the fact that readings of the border differed substantially in composition than the samples returned from Area X. No theories, however, were confirmed.
Initially, the Border sat at least fifty miles (80 km) away from the Southern Reach, before expanding at the end of Authority to cover at least the building and everything within sight of it[1], its new size unknown.
Size and expansion[]
In Annihilation and Authority, as revealed by Control, the size of the Border was as follows: Taking the Lighthouse as reference, it spanned roughly seventy miles (113 km) inland, one mile (1.6 km) out to sea, and forty miles (64 km) east and west. In height, it extended down to just above the stratosphere, and up to just under the stratosphere[2]. Thus, the total height of the border is likely around 98 miles (158 km).[3]
At the end of Authority, however, the border expanded significantly, growing to encompass the Southern Reach and everything in sight of its building. It remains unclear how long the expansion continued, or how much ground it covered.
Known openings[]
There are two known entrances to Area X. The primary entrance takes the shape of a massive arched door twenty feet high and twelve feet wide (6 by 3.7 meters) that emits a beautiful, spiraling white light. This door is invisible when looked at from the side, as if it is two-dimensional. This entry point was located fifty miles (80 km) from the Southern Reach facilities[4], before the border expansion in Authority. It was heavily guarded by the United States military with a series of checkpoints, armed guards with "shoot-to-kill" orders, fences, barbed wire, trenches, pits[4], and a system of lasers that would activate and kill anyone, besides expedition members, attempting to cross.
Another method, employed by Control and Ghost Bird at the end of Authority, involved a smaller, possibly temporary, door created underwater just as the border was growing to engulf them both. The mechanics of this, however, remain unknown even by the two of them, possibly being caused by Ghost Bird being a doppelgänger and thus "welcome" in Area X.
Despite there being so many security measures in place, the doppelgangers of the Eleventh and Twelfth expeditions were able to cross the Border from Area X back into the wider world. It is currently unknown how they were able to return undetected.
In Annihilation and Acceptance, The biologist and Control individually encountered shimmering white doors at the bottom of the tower. The biologist also saw this shimmering door when looking back towards the Border as she walked into Area X. It's possible this door is an exit from Area X or a portal to somewhere else.
Crossing the Border[]
All expeditions must cross the Border in order to gain entry to Area X. Different expedition members have different experiences of crossing, but in general, it takes between three to ten hours.
Despite the crossing area being so large, the expedition members must crawl on their stomachs while carrying their backpacks and must stick to the middle, avoiding the sides. It is never explained what happens if they touch the sides, but it's likely they would experience something similiar to the rabbits in the rabbit experiment.
The act of crossing the Border is physically and mentally disorienting, requiring the leader of each expedition to hypnotize the other members so that they do not remember the experience. The expedition leader then puts themselves under self-hypnosis. Leaders report feeling as if they are crawling through a wide open field and as if they are crawling on the edge of a cliff without guard rails, all at once.